It’s a fungus that only attacks roses. It lives on the canes and on fallen dead leaves in the winter and then spreads to the leaves when the weather is moist or wet. Problem in areas where the summers are warm and humid.

Control

Buy resistant varieties

Rake up dead leaves, good sanitation.

Be aggressive as soon as you see it. Apply a baking soda spray (2 tablespoons baking soda, 1 gallon of water and optional sticker/spreader of 1 teaspoon liquid car wax or vegetable oil.)